In the dual-source CT, by way of the simultaneous use of two emitter detector systems, x-ray quanta on the object to be measured are scattered by an x-ray emitter of a first emitter detector system into a detector of a second emitter detector system which is arranged angularly offset on the same gantry and vice versa. In order to prevent artifacts, this so-called transverse scattering must be corrected with suitable methods. To this end, the knowledge of the profile of the scattered beam intensity is needed in each projection.
To this end, the following 3 methods are essentially known:
(i) the model-based estimation of the scattered radiation from sinogram data, based on data which chronologically predates the point in time of the correction.
(ii) the measurement of the scattered radiation with the aid of dedicated sensors.
(iii) the alternate blanking of the respective primary radiation and direct measurements of the transverse scattering during the blanking phase.
It is common to all three variants that the scattered beam correction of the raw data is implemented once prior to calculating a CT image into the raw data. Methods of model-based estimation and measurement with dedicated sensors are used successfully in commercially available dual-source systems.
Strictly speaking the method of model-based estimation (i) assumes that when scanned in sequential mode, the object does not change or only changes insignificantly during a spiral scanning along the feed direction (z-direction). With an increased z-coverage of the detector with simultaneously high values for the pitch during spiral scanning, this estimation of the scattered beam profile is however increasingly incorrect. On the other hand, with these methods disclosed for instance in the publication DE 10 2007 014 829 B3 (the entire contents of which are hereby incorporated herein by reference) for classifying the object tangents using scattered radiation, ambiguities appear in the determination of the scattering surface, which result in an inadequate estimation of the scattered beam profile appearing on the object.
The method of measuring the scattered radiation using dedicated sensors (ii) on the one hand requires additional hardware, which may determine a not insignificant part of the manufacturing costs of a dual-source device. On the other hand, with this method, the scattered beam profile is not measured directly at the z-position at which the correction is then implemented. Any variations in the z-profile of the scattered radiation are therefore not detected or only in a spatially sub sampled fashion. An increasing z-coverage of the detector results in an increasingly incorrect correlation of the measured scattered beam profile with the necessary correction of data.
The third method of alternate blanking of the respective primary radiation and direct measurements of the trans-verse scattering during the blanking phase (iii) nevertheless does not suffer from this potentially inadequate correlation of the data in the z-direction, but the data here can in turn not be scanned as precisely as may be required in the angular direction, which results in corresponding aliasing errors both in the scattered data and also in the primary data. In addition, the image noise increases by the blanking of the primary radiation. The dose efficiency is also less, since, in reality, the switch-on and off phases of the emitter cannot be arbitrarily short. A rapid blanking of the x-ray radiation not least makes high demands on the x-ray emitter and high voltage generator of a CT device.